The present invention relates to absorbents that satisfactorily absorb body fluids and the like, absorbing products, such as disposable diapers, incontinence pads, and sanitary napkins, based on the absorbents, and water-absorbing resins preferably used for the absorbents and absorbing products.
Conventional absorbing products, such as disposable paper diapers, incontinence pads, and sanitary napkins, has a basic structure of a top sheet, a back sheet, and an absorbent interposed between these sheets. The top sheet is permeable to liquid and touches the body of the user when he/she is in the product, thereby allowing body fluids to pass through it to the absorbent. The back sheet is impermeable to liquid and lies outside when he/she wears the product, thereby preventing the body fluids (water-based liquids) absorbed by the absorbent from leaking.
In such a situation, most back sheets, since having liquid impermeability, exhibit low gas permeability too. Therefore, vapor discharged from the body, as well as body fluids that have been absorbed and later evaporated due to body heat, cannot escape and accumulate between the absorbing product and the body. Consequently, a high humidity condition occurs where the absorbing product is being worn and causes a humid, sticky, or other unpleasant feel. If the product is worn for an extended period of time, it may cause a rash or other irritation to develop on the skin.
Accordingly, attempts have been made in the prior art to prevent an unpleasant feel and thereby improve the sense of comfort when the user is in the absorbing product, which are basically classified into the following three kinds of techniques.
The first kind is to improve on the back sheet. According to the technique, a high humidity condition is prevented from occurring by imparting ventilating properties to the back sheet with its liquid impermeability remaining intact.
Specific examples include Japanese Laid-Open Patent Application No. 58-149303/1983 (Tokukaisho 58-149303; published on Sep. 5, 1983) disclosing use, as the back sheet, of a liquid impermeable moisture-prevention sheet prepared by kneading polyolefin, a filling agent, and other materials, extending the mixture into a film, and forming microscopic pores; Japanese Laid-Open Patent Application No. 11-106536/1999 (Tokukaihei 11-106536; published on Apr. 20, 1999) disclosing use, as the back sheet, of a moisture permeable film prepared by extending a resin composition which contains minuscule particles of a filling agent and which is blended with cellulosic particles and forming pores in the extended resin composition; and Japanese Laid-Open Patent Application No. 11-106537/1999 (Tokukaihei 11-106537; published on Apr. 20, 1999) use, as the back sheet, of a moisture permeable film prepared by extending a resin composition which contains minuscule particles of a filling agent, which melts at a molding temperature, and which is blended with non-flowing polyolefin particles and forming pores in the extend resin composition.
The second kind is to provide a moisture absorbent. According to the technique, generated vapour is removed using moisture absorbent to prevent a high humidity condition from occurring. A specific example, among others, is Japanese Laid-Open Patent Application No. 6-218007/1994 (Tokukaihei 6-218007; published on Aug. 9, 1994) disclosing provision of a water-absorbing resin or other moisture absorbent in the absorbing product to prevent evaporation of body fluids.
Lastly, the third kind is to improve on the structure of the absorbing product. According to the technique, a high humidity condition is prevented from occurring by improving on the structure of the absorbing product and thereby preventing generation of vapor and encouraging dispersion of vapor. A specific example, among others, is Japanese Laid-Open Patent Application No. 11-99165/1999 (Tokukaihei 11-99165; published on Apr. 13, 1999) disclosing reducing the area where the absorbent touches the body to a minimum extent and creating a ventilating space between the absorbent and the body when the use is in the product by the use of a material that coats the absorbent.
All the foregoing techniques fall short of offering a sufficiently improved comfort when the user is in the absorbing product. Specifically, to prepare an absorbing product that provides a high level of comfort when worn, at least two problems need be addressed concurrently: (1) Absorbency under load must be raised to reduce xe2x80x9cwet back,xe2x80x9d i.e., amounts of body fluid undesirably released after absorbed (elimination of a sticky feel). (2) The absorbent per se must possess improved ventilating properties (elimination of a humid feel). None of the techniques offers a sufficient level of solution to these problems.
Particularly, the absorbing product is spread over an expanded area since it absorbs the body fluid when it touches the body. Therefore, the absorbent as such, as well as the back sheet, acts as an air-tight separation wall having absorbed the body fluid and obstructs dispersion of vapor accumulated between the absorbing product and the body.
Without addressing the problem of the absorbent having absorbed the body fluid, etc. acting as an air-tight separation wall, no techniques, including the first to third techniques, can successfully solve problem (2) (elimination of a humid feel) and offer a sufficiently improved sense of comfort to the user in the absorbing product.
We, the inventors of the present invention, have diligently worked to solve these problems. Particularly, attention has been paid to the absorbent acting as an air-tight separation wall once it absorbs a body fluid or the like and changes to a wet state. As a result, we have found that the user feels more comfortable in the absorbing product if the absorbent has improved ventilating properties in a wet state and concurrently, the absorbed body fluid xe2x80x9cwets backxe2x80x9d the user only in reduced amounts, which has led to the completion of the invention.
In order to solve these problems, the absorbent in accordance with the present invention is characterized in that it has a 24 g/g or more absorbency under a 2.0 kPa load to physiological salt solution and a 50 kPaxc2x7sec/m or less ventilation resistance under a 4.9 kPa load in a wet state. The absorbent preferably contains a 40 weight percent or more water-absorbing resin and has a maximum basis weight of 700 g/m2 or less.
Further, the absorbing product in accordance with the present invention is characterized in that it includes: an absorbing layer containing the absorbent; a liquid permeable sheet; and a liquid impermeable sheet having a ventilation resistance of not less than 1 kPaxc2x7sec/m and not more than 50 kPaxc2x7sec/m, the absorbing layer being disposed between the two layers.
In the conventional absorbent and absorbing product, no consideration is given to the ventilating properties of an absorbent per se in a wet state. Therefore, typical absorbents exhibit a high ventilation resistance of 100 kPaxc2x7sec/m or more in a wet state, which means that the absorbent, in practice, has no ventilating properties. Absorbents do exist that exhibit appreciable ventilating properties in a wet state. However, they cannot retain sufficient amounts of water-based liquid under load or sufficiently reduce the wet back of absorbed water-based liquid.
In contrast, in the present invention, the arrangement ensures excellent ventilating properties even when the absorbent or absorbing product is in a wet state and can prevent increases in wet back of absorbed water-based liquid. This prevents the absorbent from acting as an air-tight separation wall and a high humidity condition from building up between the body and the absorbent or the absorbing product, as well as enables the absorbent to sufficiently retain absorbed water-based liquid even under load. Hence, a humid feel and a sticky feel can be eliminated concurrently, enhancing the comfort the user feels when he/she is in the absorbing product.
An example of water-absorbing resin suitably used for the absorbent and absorbing product in accordance with the present invention is the one characterized in that it: possesses a ventilation resistance of 250 kPaxc2x7sec/m or less under a 4.9 kPa load in a wet state; has a 32 g/g or more absorbency under no load to physiological salt solution and a 32 g/g or more absorbency under a 2.0 kPa load to physiological salt solution; and is shaped in particles with a weight mean particle diameter of 430 xcexcm or more.
Another example of water-absorbing resin suitably used for the absorbent and absorbing product in accordance with the present invention is the one characterized in that it: has a 250 kPaxc2x7sec/m or less ventilation resistance under a 4.9 kPa load in a wet state and a 34 g/g or more absorbency under no load to physiological salt solution; and comprises 18 weight percent or less water-soluble components.
A further example of water-absorbing resin suitably used for the absorbent and absorbing product in accordance with the present invention is the one characterized in that it: has a 250 kPaxc2x7sec/m or less ventilation resistance under a 4.9 kPa load in a wet state and a 34 g/g or more absorbency under a 2.0 kPa load to physiological salt solution; and comprises 18 weight percent or less water-soluble components.
In other words, the water-absorbing resin in accordance with the present invention satisfies an essential condition that it has a 250 kPaxc2x7sec/m or less ventilation resistance under a 4.9 kPa load in a wet state and one of first, second, or third groups of conditions: The first group of conditions is such that the water-absorbing resin has a 32 g/g or more absorbency under no load to physiological salt solution and a 32 g/g or more absorbency under a 2.0 kPa load to physiological salt solution and is shaped in particles with a weight mean particle diameter of 430 xcexcm or more. The second group of conditions is such that the water-absorbing resin has a 34 g/g or more absorbency under no load to physiological salt solution and contains 18 weight percent or less water-soluble components. The third group of conditions is such that the water-absorbing resin has a 34 g/g or more absorbency under a 2.0 kPa load to physiological salt solution and contains 18 weight percent or less water-soluble components.
By using such a water-absorbing resin that satisfies these parameter requirements, the absorbent shows a ventilation resistance of 50 kPaxc2x7sec/m or less in a wet state and a 24 g/g or more absorbency under a 2.0 kPa load. The absorbing product based on the absorbent does not give a humid or sticky feel to evaluators, meaning that it creates a low humidity condition when worn. The water-absorbing resin can be thus suitably used for the absorbent and the absorbing product in accordance with the present invention.
Particularly, as mentioned earlier, the absorbent in accordance with the present invention preferably contains the water-absorbing resin with a ratio of 40 weight percent or more. This ensures that the absorbent does not act as an air-tight separation wall, prevents a humid feel and a sticky feel concurrently, and further enhances the comfort the user feels when he/she is in the absorbing product.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.